The present invention relates to a disc drive apparatus capable of recording or playing back data onto and from an optical-disc recording medium. More particularly, the present invention relates to a disc drive apparatus for recording or playing back data onto and from an optical-disc recording medium having a plurality of signal surface areas outputting detected information as reflected detection signals with different amplitudes.
A DVD (Digital Versatile Disc or Digital Video Disc) is known as disc media. A DVD has been developed as the so-called DVD-ROM and the so-called DVD-RAM, which have been becoming popular. A DVD-ROM is a playback-only DVD onto which data cannot be recorded. On the other hand, a DVD-RAM is a DVD that allows data stored thereon to be rewritten. Data is recorded onto a DVD-RAM by creation of recording pits by adopting the so-called phase-change technique.
In accordance with a track format of a DVD-RAM, a recording track which data is recorded onto and played back from is divided into units each called a sector in the circumferential direction. Each sector is a recordable area with the beginning thereof serving as a header area.
A header area is area in which data is recorded as a pit array. On the other hand, the remaining recordable area is an area used for recording data that can be rewritten by adoption of the phase-change technique. That is to say, the header area adopts a recording technique different from that adopted for the recordable area. Thus, the optical quantity of a laser beam radiated to the header area and then reflected thereby is also different from the optical quantity of a laser beam radiated to the recordable area and then reflected thereby.
Roughly speaking, the header area includes 4 recorded addresses, namely, PID1, PID2, PID3 and PID4 which are each a physical address. Pit arrays of PID1 and PID2 are placed at locations shifted from the center line of a groove track by ½ track pitches in the direction toward a circumference on the outer side. On the other hand, pit arrays of PID3 and PID4 are placed at locations shifted from the center line of the groove track by ½ track pitches in the direction toward a circumference on the inner side. That is to say, 2 track positions in the header area are separated from a track position in the recordable area by ½ track pitches in the radial direction of the disc toward circumferences on the outer and inner sides respectively. It should be noted that data is recorded onto a DVD-RAM by adoption of the so-called land-groove recording technique whereby data is recorded onto both a land and a groove.
For the reason described above, in a disc drive apparatus for a DVD, during an operation to play back data from the DVD, for example, it is necessary to hold a tracking servo control while a laser beam tracing a track is passing through a header area. That is to say, if the tracking servo control is held while the laser beam tracing a track is passing through a header area, there is generated no shift from the track of the recordable area in the trace position of the laser beam in the tracking direction.
In addition, since a header area adopts a recording technique different from that adopted by a recordable area as described above, it is necessary to modify a variety of parameters or the like of a playback-signal-processing circuit.
In order to hold the tracking servo control and to modify a variety of parameters or the like of the playback-signal-processing circuit with a proper timing, that is, while the laser beam tracing a track is passing through a header area, as described above, it is also necessary to detect the proper timing with which the laser beam tracing a track is passing through the header area.
It should be noted that, in the following description, the detection of the proper timing with which the laser beam tracing a track is passing through a header area is also referred to simply as header detection.
The conventional header detection is explained by referring to FIG. 14 and FIGS. 15A to 15C as follows.
FIG. 14 is a diagram showing a typical configuration of a header detection circuit. An optical pickup 101 radiates a laser beam for playing back data to a disc 1 serving as a DVD. The radiated laser beam is reflected, and the reflected laser beam is received as an optical-reception signal by a photo detector not shown in the figure in a detection process. The optical-reception signal is then supplied to a push-pull signal generation circuit 102. The push-pull signal generation circuit 102 uses the optical-reception signal supplied thereto to generate a push-pull signal PP. Roughly speaking, the push-pull signal PP is generated as a differential signal between 2 detection signals detected by 2 optical-reception areas which are obtained as a result of dividing the photo detector by a dividing line oriented in the track direction.
The push-pull signal PP output by the push-pull signal generation circuit 102 is supplied to a low-pass filter 103 for eliminating harmonic components from the push-pull signal PP so as to produce a smooth envelop waveform. A push-pull signal PPL passing through the low-pass filter 103 is split, being supplied to comparators 104 and 105.
In a process to correctly detect a header, for example, the push-pull signal PPL has a waveform shown in FIG. 15A. As shown in the figure, in a header period corresponding to a laser beam's passing through a header area, the waveform is divided into a preceding half period and a succeeding half period, which correspond to detection of pit arrays of PID1 and PID2 and detection of pit arrays of PID3 and PID4 respectively. As is obvious from an earlier description, the pit arrays of PID1 and PID2 are separated from the pit arrays of PID3 and PID4 by 1 track pitch. As shown in the figure, the waveform of the preceding half period is an inverted one of the waveform of the succeeding half period. To be more specific, in the case shown in the figure, the waveform of the preceding half period has a positive polarity while the waveform of the succeeding half period has a negative polarity. It should be noted, however, that the waveform of the preceding half period can also have a negative polarity while the waveform of the succeeding half period has a positive polarity. The polarities of the waveform of the preceding half period and the waveform of the succeeding half period are determined by whether the recording track of the recordable area following the head area is a land or a track.
In the header detection circuit shown in FIG. 14, the push-pull signal PPL passing through the low-pass filter 103 is split, being supplied to the comparators 104 and 105 as described above each to be compared with a reference value. The reference value used for comparison in the comparator 104 is a predetermined threshold value th1 set for the detection waveform in the positive-polarity direction. On the other hand, the reference value used for comparison in the comparator 105 is a predetermined threshold value th2 set for the detection waveform in the negative-polarity direction. The threshold values th1 and th2 are each a constant determined in advance. In FIG. 15A, the levels of the threshold values th1 and th2 are each indicated by a dashed line.
The comparator 104 compares the push-pull signal PPL with the threshold value th1. If the absolute value of the level of the push-pull signal PPL exceeds the absolute value of the threshold value th1, a detection signal DT·h1 set at an H level is output as shown in FIG. 15B.
By the same token, the comparator 105 compares the push-pull signal PPL with the threshold value th2. If the absolute value of the level of the push-pull signal PPL exceeds the absolute value of the threshold value th2, a detection signal DT·h2 set at an H level is output as shown in FIG. 15C.
As described above, the detection signals DT·h1 and DT·h2 are set at a high level to indicate the preceding half period and the succeeding half period of the header period respectively. In this way, the preceding half period and the succeeding half period of the header period are detected in processing called the header detection cited above.
As described above, however, the header detection is carried out by comparing a push-pull signal PPL with a fixed threshold value. It is thus quite within the bounds of possibility that the header detection is carried out incorrectly due to the fact that the threshold value is fixed. This problem is described by referring to FIGS. 16A to 16C.
In the case of an abnormality such as a detrack caused by disc eccentricity, vibration of the position of a laser-beam spot or a defect caused by dirt or dust stuck on the surface of the disc, for example, an unnecessary offset may unavoidably be added to the push-pull signal PPL itself as shown in FIG. 16A as an effect of the abnormality. As shown in the figure, the offset is inadvertently added to the push-pull signal PPL so that the level of the push-pull signal PPL is raised.
When the push-pull signal PPL with such an offset added thereto is compared with the threshold value th1 and the threshold value th2 by the comparator 104 and the comparator 105 respectively, to put it concretely, the absolute value of the push-pull signal PPL shown in FIG. 16A does not exceed the absolute value of the threshold value th2 during the header period's succeeding half period in which the absolute value of a normal push-pull signal PPL would naturally exceed the absolute value of the threshold value th2. Thus, there is resulted in a state in which the signal DT·h2 cannot be output at the H level as shown in FIG. 16C.
Since an offset is inadvertently added to the push-pull signal PPL to raise the level in the positive direction as shown in FIG. 16A, on the contrary, the signal DT·h1 exceeds the level of the threshold value th1 not only during the header period's preceding half period in which a normal push-pull signal PPL can exceed the threshold value th1, but also, for example, a period A outside the header period. Thus, the signal DT·h1 is set at the H level also during the period A outside the header period as shown in FIG. 16B, causing incorrect header detection.